JP2606972B2 - Electromagnetic shielding materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shielding material such as a shielding container for shielding an electromagnetic field used in connection with electric equipment and electronic equipment.

[0002]

2. Description of the Related Art As the performance of electric and electronic devices has become higher and their use has expanded rapidly, the magnetic fields used by these devices have become larger and denser, while protecting operating rooms, test rooms or human bodies from magnetic fields. Or to prevent interference between electronic devices due to magnetism,
Active shielding for shielding a magnetic field from an apparatus and passive shielding for shielding a magnetic field that intrudes into an apparatus are performed, and a magnetic shielding material is used. For example, in an imaging apparatus using nuclear magnetic resonance (NMR), a high magnetic field is required by a superconducting magnet or the like, so that a magnetic flux leaking outside the measurement region increases. Therefore, a magnetic shield is provided to reduce the leakage magnetic field to the environment. In addition, a weak magnetic environment is required for biomagnetic research using SQUID or the like, and a magnetic shield for shielding external magnetic noise is performed. In such a magnetic environment, not only a DC magnetic field but also a substantially AC magnetic field shield is important.

[0003] In order to avoid such magnetic interference, a soft magnetic material such as iron is used as a shielding material. For example, magnetic shielding is performed using a thick iron plate or the like utilizing nuclear magnetic resonance (NMR). In addition, in order to shield a room, in response to demands for miniaturization and weight reduction, a magnetic ribbon such as an electromagnetic steel plate, permalloy, or amorphous has recently been used. In general, a material having high magnetic permeability is used for the magnetic shield. However, in practice, an appropriate material needs to be selected depending on the type and strength of the magnetic field to be shielded. In general, a magnetic material that effectively shields a DC magnetic field is used for the magnetic shield, and an electromagnetic wave shield is made of a conductive material and is effectively shielded by eddy current. However, electromagnetic shielding with conductive materials is effective at least several tens of kHz.
In the above-mentioned AC magnetic field, the AC magnetic field from DC to several tens of kHz has not been considered so far, and a ferromagnetic material is used in the same concept as a DC magnetic field shield.

[0004] Since amorphous magnetic materials have high magnetic permeability and are available as thin ribbons, it is easy to manufacture shields in the form of cylinders and boxes, and they are used as cast.
However, even if such a shield is shielded in an AC magnetic field environment of several tens of kHz or less, the shield may not be effectively shielded. That is, there are cases where the shielding performance does not correspond to the magnetic permeability characteristic that is usually measured by magnetism.

The AC magnetic shield is generally used to shield an external magnetic field having a magnetic field strength of several tens of kHz or less and a magnetic field strength of several hundred mG or less. Therefore, permalloy having a high magnetic permeability even though the magnetic flux density is low can be considered for the shield in such a weak magnetic field.
However, permalloy is sensitive to strain and significantly reduces magnetic permeability. Annealing after processing is strict and difficult for a long time at very high temperature in a special atmosphere, and deformation after annealing must be considered. Further, permalloy is expensive, increases the cost of magnetic shielding, and has various problems as a shielding material.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a light-weight and inexpensive shielding material exhibiting a good shielding effect against an alternating magnetic field.

[0007]

According to the present invention, an Fe-based amorphous ribbon is used as a magnetic material for magnetic shielding, a plurality of sheets are laminated to form a shield body, and then annealed at a temperature lower than crystallization without a magnetic field. Provide materials with excellent magnetic shielding properties, especially those with varying magnetic field shielding properties of several kHz or more. This amorphous ribbon shield is composed of two or more laminated layers.
Usually, it is used as a laminate of 2 to 25 sheets. Even if only a single layer of an ordinary amorphous ribbon is used as a shield, no remarkable difference in shielding performance is observed.

The shield body may be processed like a cylinder, or may be used in a flat shape like a panel. The laminate may be just stacked, but for ease of handling,
It may be partially or wholly joined. However, when joining, it is important to minimize distortion.
If a wide shield is required, it may be joined by various previously announced methods, or it may be merely superposed.

A feature of the shield material of the present invention is that annealing is performed without a magnetic field in order to effectively shield an AC magnetic field, and annealing is not performed in a magnetic field that is normally performed in order to improve magnetic permeability. is there. Annealing conditions are such that the amorphous material is not crystallized, and a temperature of usually 300 ° C. to 400 ° C. is selected. The atmosphere is preferably non-oxidizing, but may be in the air if it is annealed for a short time.

The amorphous alloy material used in the present invention comprises:
It is an Fe-based alloy usually represented by Fea-Xb-Mc. X represents one or more of 3d and 4d elements, the content of which is 0 to 40 at%, M is a metalloid element, and B,
At least one of Si, C, P, and Ge is shown. Its content is in the range of 10 to 30 at%. The balance is Fe and inevitable impurities. Co-based amorphous may be used, but is very expensive and is not suitable for use as a large-area shield member.

When a magnetic shield is used, a material having a high magnetic permeability is required for selecting a shield material. The magnetic shielding property is related to the magnetic permeability of the material. For example, in the case of an infinite cylindrical shield, the shielding property S is a value obtained by dividing the product of the magnetic permeability μ and the plate thickness t by the cylindrical diameter. The shielding property S also increases. Here, the shielding property S is a value obtained by dividing the magnetic field strength He to be shielded by the shielded magnetic field strength Hi.

Therefore, annealing is performed in a magnetic field to improve the magnetic permeability. However, it has been found that in the shield body in which the magnetic permeability is improved in this way, the shielding performance under an AC magnetic field is not improved corresponding to the improvement in the magnetic permeability. In other words, it is clarified that the microscopic magnetic structure of the magnetic material is more important than the magnetic permeability under the AC magnetic field, and a method that can dramatically improve the shielding performance of the shield by annealing in a magnetic field-free state. It has been developed.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments.

Example 1 A cylindrical shielded container having an inner diameter of 60 mm and a length of 200 mm was set to 25 μm.
Fe-Si-B amorphous ribbons of one layer, three layers and 6 layers
It consisted of layers. The cylindrical container was subjected to as-cast (A) and annealing in a DC magnetic field of 15 Oe (B) and non-magnetic annealing (C) to obtain a shield body. Annealing conditions are all 350 ° C 30
Minutes. The shielding performance of the shield was measured under an AC magnetic field of up to 20 kHz. As for the shielding performance, tangential shielding property ST (an external AC magnetic field is applied perpendicularly to the length of the shield cylinder) was evaluated (FIG. 1). In the figure, 1: shield body, 2: magnetic field sensor, 3: measuring instrument, 4: external alternating magnetic field are shown. The magnetic permeability of the shield was measured in parallel (in the circumferential direction) with the shield magnetic field to obtain the magnetic permeability. Table 1 shows the relationship between the shielding performance and the magnetic permeability. It is clear from Table 1 that the shield according to the present invention has very good shielding performance despite its low magnetic permeability.

[0015]

[Table 1]

Example 2 A cylindrical shield body having an inner diameter of 60 mm and a length of 150 mm was manufactured by a magnetic field-free annealing using six Fe-Si-B amorphous ribbons according to the present invention. On the other hand, a similar cylinder was made of 78% Ni permalloy (PC) with a plate thickness of 0.2 mm and annealed at 1100 ° C.
It was a shield body. The shield performance was evaluated by the same measurement method as in Example 1. Table 2 shows the results. External AC magnetic field is 3
The frequency was set to 0 mG and the frequency was set to 10, 20 kHz.

[0017]

[Table 2]

The shield according to the present invention has a magnetic shielding property equal to or higher than that of permalloy, which is a representative of high permeability materials.

[0019]

According to the magnetic shield material of the present invention, the shield effect can be greatly improved in an AC magnetic field environment. As a result, cost reduction and weight reduction were achieved, and workability and handling, including annealing, were greatly improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of shield performance measurement.

Claims (1)

(57) [Claims] 1. An electromagnetic shielding material obtained by laminating two or more amorphous magnetic ribbons, wherein the material is annealed in a temperature range equal to or lower than the crystallization temperature of the amorphous magnetic ribbon and without a magnetic field.